Offset printing is a printing method using repulsion of an oily ink composition for offset printing (hereinafter, appropriately abbreviated to “ink composition” or “ink”) from water. Unlike letterpress printing using a printing plate with projections and depressions, offset printing uses a printing plate having lipophilic image portions and hydrophilic non-image portions with no projections or depressions. In printing using this printing plate, dampening water is initially brought into contact with the printing plate to form a water film on the surfaces of the non-image portions, and an ink composition is fed to the printing plate. The fed ink composition then repels from the non-image portions covered with the water film, but adheres to the lipophilic image portions. An image of the ink composition is thereby formed on the surface of the printing plate, and is sequentially transferred onto a blanket and a paper sheet to complete printing.
Besides offset printing using dampening water described above, dry offset printing has practically been performed which uses a printing plate on which non-image portions of silicone resin are formed. In this printing method, an ink composition repels from the silicone resin, instead of dampening water, to form the non-image portions. Except for such features, dry offset printing is also considered a printing method having features shared with offset printing using dampening water. Accordingly, the concept represented by the term “offset printing” used herein includes not only a printing method using dampening water but also a dry printing method.
Printed materials prepared by offset printing cannot be fed to subsequent steps or be circulated as commercial products until the ink composition adhering to the surfaces of the printed materials is sufficiently dried, because an undried ink composition causes adhesion of ink onto the rear surfaces of stacked printed materials or adhesion thereof to fingers when those printed materials are touched with fingers. Accordingly, a step of drying the ink composition adhering to the surface of the printed material is needed after offset printing is performed. To accomplish such a step in a short time, printing methods using active energy ray-curable ink compositions have been intensively performed in recent years. An ink composition of this type comprises a polymerizable compound, such as a monomer or oligomer, and a polymerization initiator, which polymerizes the polymerizable compound when irradiated with active energy rays, for example, ultraviolet (UV) light or electron beams. When the undried surface of a printed material printed with this ink composition is irradiated with an active energy ray, the polymerizable compound contained therein is converted into a polymer. As a result, the ink composition present on the surface of the printed material instantaneously turns to a non-sticky (i.e., dried) coating. Ink compositions used in combination with such a drying method have been proposed (see Patent Literature 1 and 2, for example). Examples of the active energy ray used in this step include UV light and electron beams. UV light is often selected in consideration of cost and handling of the apparatus.
Activities for reducing environmental loads have been developed in a variety of industries and businesses, and such activities have been promoted also in the printing industry for a reduction in environmental loads. In such a context, in the case where UV light is used in the printing method using the active energy ray-curable ink composition described above, conventional UV lamps having high power consumption and generating ozone by short wavelength UV light are being replaced with UV LED lamps and low-output UV lamps which have lower power consumption and reduce the generation of ozone.